In a selective call communication system modern base stations use accurate frequency deviation modulators for transmission of M-ary frequency shift keyed (FSK) frequency modulated (FM) signals for transmission of outbound messages to portable transceivers. Normally these base stations service very large numbers of subscriber users who are accustomed to low cost subscriber units providing reliable transmission of selective call messages. For this reason, the base stations use accurate timebase systems to generate accurate frequency deviation levels during outbound message transmission to the portable transceivers. The accuracy of the transmitted signals provides a means for accurate message reception by the portable transceivers with relatively inexpensive conventional receiver hardware.
The inbound messages transmitted by the portable transceivers provide an important means for the base stations to determine the reliability of the message transaction. For this reason, accurate inbound transmission is also important. Since the proliferation of portable transceivers is much higher than that for the base stations and since subscriber units are expected to be inexpensive by the common user, it is unfeasible and too costly to consider a transmission architecture design similar to the base stations for the portable transceivers.
Thus, what is needed is a cost effective method and apparatus in the portable transceiver for controlling frequency deviation levels during inbound transmission while maintaining the accuracy needed for the base stations to reliably receive the message.